The comparative mapping anchor tagged sequences (CATS) primers are a set of conserved sequences of Type I genetic markers (coding genes) that are being used to generate genetic maps for a variety of mammalian species. While these genes have been localized cytogenetically in humans, strong estimates of gene order and distance are not available for the CATS in that species as they are in the mouse genome. Radiation hybrid (RH) mapping is the current means to generate stronger gene order and distance estimates for humans and several other species. The CATS have been shown to amplify specific products in a wide variety of mammalian species, including humans, but the RH locations for only 199 of the 318 CATS genes were found in public databases. This lack of complete data on the localization of the CATS on the human gene map makes comparisons with other species difficult, thus inhibiting the human species from the benefit of the genome and biology knowledge available for other animals. In order to determine RH map locations for the remaining CATS, we have screened the GeneBridge 4 panel of RH cell lines using CATS primers or improved primers that were specifically designed to amplify the human sequences represented by the CATS. This involved amplification by polymerase chain reaction (PCR) of DNA from each of the 93 cell lines from the panel. Duplicate analyses were performed in order to assess the degree of reproducibility of the assays. Scoring of PCR results was conducted according to the standards of the Massachusetts Institute of Technology Whitehead Institute (MIT WI). The data were compiled in vector format and analyzed by the MIT WI RH Server. At present, 55 of the CATS genes for which RH positions were not found in the databases have been mapped in this laboratory. In each case, the RH map position is consistent with the previously reported cytogenetic localization. We intend to complete the genetic localization of the entire set of CATS in the human RH panel. Since a majority of the genome projects from other species are using the same set of CATS for Type I gene mapping, this project will improve the human gene map and will allow effective comparisons across species for use in diverse biological and genetic fields.